This invention relates to a method and apparatus for cutting sheet material, such as garment or upholstery fabrics, and deals more particularly with an apparatus and method utilizing a cutter with a reciprocating cutting blade.
The type of apparatus with which this invention is concerned may be used for cutting garment or upholstry pieces and the like from either a layup of sheet material arranged in vertically stacked relationship or from a single sheet which is spread out in a generally flat condition prior to the cutting operation. The supporting surface for the sheet material is generally arranged horizontally and the cutter is supported above the sheet material for movement in two coordinate directions parallel to the supporting surface to enable the cutting blade to be moved along any desired line of cut in the sheet material. As the cutter is moved along the desired line of cut, the cutting blade engages the sheet material and is reciprocated to execute the actual cutting action.
It is common to cut spread sheet material with a reciprocating-blade cutter so that the blade cuts with a slicing action as the blade reciprocates perpendicularly to the plane of the sheet material. The cutting blade has a leading cutting edge parallel to the axis of reciprocation and the fabric is cut as the leading edge is urged progressively into the sheet material along a desired line of cut.
In some circumstances, it is desirable to dither the blade, that is, to intermittently and rapidly rotate the blade by preselected amounts about a position in alignment with or tangent to the cutting path without transversing a substantial segment of the cutting path during rotation. Dithering is desirable when, for example, the cutting blade is experiencing unbalanced lateral loading from limp sheet material due to the absence or lack of lateral support at one side of the blade near the edges of the layup or at closely adjacent cuts, or when the blade loading is unbalanced because of anisotropic material characteristics. The abrupt rotation as the blade advances cuts the material in a stepwise fashion and relieves the loading and blade stress caused by the material.
U.S. Pat. No. 4,133,234, having the same inventor as this invention, describes a computer-controlled method for obtaining abrupt rotational movements, or dither, of a reciprocating blade. In that patent, a .theta.-servomotor adapted to rotate the blade about a vertical axis produces blade dither in response to command signals from a control computer. In order to produce blade dither, however, several cutting assembly elements, including the sleeve in which the blade reciprocates, the presser foot, and selected rotational drive components, must be abruptly rotated in conjunction with the blade. Necessary movement of these several cutting elements contributes to a high dither load on the .theta.-servomotor, and, accordingly, a high level of electrical power is required. If the dither load is so high that the .theta.-servomotor is excessively large in size or a second servomotor is required to perform the dither function, the servomotor weight, entirely supported by the movable carriages, hinders rapid response of the carriages to movement commands.
It is, therefore, an object of this invention to provide a reciprocating cutter for sheet material which provides blade dither with less power consumption than is required by previous cutters.